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Surface treatment is an effective method to improve the photoelectrochemical (PEC) performance of photoelectrodes. Herein, we introduced a novel strategy of surface sulfurization to modify hematite (α-Fe2O3) nanorods grown in an aqueous solution, which triggered encouraging improvement in PEC performances. In comparison to the solution-grown pristine α-Fe2O3 nanorod photoanode that is PEC inefficient and always needs high temperature (>600 °C) activation, the surface sulfurized α-Fe2O3 nanorods show photocurrent density increased by orders of magnitude, reaching 0.46 mA cm−2 at 1.23 V vs. RHE (reversible hydrogen electrode) under simulated solar illumination. This improvement in PEC performances should be attributed to the synergy of the increased carrier density, the reduced surface charge carrier recombination and the accelerated water oxidation kinetics at the α-Fe2O3/electrolyte interface, as induced by the incorporation of S ions and the formation of multi-state S species (Fe-Sx-Oy) at the surface of α-Fe2O3 nanorods. This study paves a new and facile approach to activate α-Fe2O3 and even other metal oxides as photoelectrodes for improved PEC water splitting performances, by engineering the surface structure to relieve the bottlenecks of charge transfer dynamics and redox reaction kinetics at the electrode/electrolyte interface.

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (nos. 51961165103 and 51672210) and the National Key Research and Development Program of China (2018YFB1502003 and 2017YFE0193900). S.S. is supported by the National Program for Support of Top-notch Young Professionals and “The Youth Innovation Team of Shaanxi Universities.”

In this study, we report a simple, effective and universal approach to fabricate homojunction structural TiO2 nanorod array photoelectrodes through doping of gradient distributed oxygen vacancies introduced by annealing in high vacuum with controlled time. The homogeneous junction structure promotes the separation and transport of photoexcited charge carriers, and the photocurrent density of TiO2 nanorod arrays annealed in high vacuum for 2 h was 20 times higher than that of the pristine TiO2 nanorod arrays. The results indicate that annealing in high vacuum with controlled time could form a homojunction structure, which is an effective approach for further enhancing the performance of UV–visible light driven photocatalytic materials for water splitting.

Given the proper band gap, low cost and good stability, hematite (α-Fe2O3) has been considered as a promising candidate for photoelectrochemical (PEC) water splitting, however suffers from the sluggish surface water oxidation reaction kinetics. In this study, a simple dip-coating process was used to modify the surface of α-Fe2O3 nanorod arrays with cobalt oxide (CoOx) and carbon (C) for the improved PEC performance, with a photocurrent density at 1.6 V (vs. reversible hydrogen electrode, RHE) increased from 0.10 mA/cm2 for the pristine α-Fe2O3 to 0.37 mA/cm2 for the CoOx/C modified α-Fe2O3 nanorods. As revealed by electrochemical analysis, thanks to the synergistic effect of CoOx and C, the PEC enhancement could be attributed to the enhanced charge transfer ability, decreased surface charge recombination, and accelerated water oxidation reaction kinetics. This study serves as a good example for improving PEC water splitting performance via a simple method.

In this study, Nb-doped α-Fe2O3 nanorod photoanodes were prepared via a facile solution-based process by directly adding NbCl5 in the aqueous solution. The morphology and electronic structure of Nb-doped α-Fe2O3 films depended strongly on the dopant concentrations. Compared with the undoped sample, the optimal Nb-doped α-Fe2O3 film showed an approximately 4-fold photocurrent increase under solar light at 1.0 V versus Ag/AgCl, and the incident photon-to-current conversion efficiency was increased by 2.5 times, reaching 13.7 % at 350 nm and 1.23 V versus RHE. The enhancement in PEC activity, as induced by moderate Nb doping, was attributed to the increased charge carrier density for promoted charge transfer ability as well as the smaller nanorod diameter for shortened charge transfer distance. However, superfluous Nb dopants would destroy the nanorod structure and greatly reduce the thickness of α-Fe2O3 films, leading to poor optical absorption and thus decreased photoelectrochemical performance.

Porous photoelectrodes show high efficiency in hydrogen production by water splitting. However, fabrication of porous nanorods is usually difficult. Here, we report a simple approach to fabricate a kind of novel porous rutile titanium dioxide nanorod array by an advanced ion implantation method using multiple-energy helium ion implantation and subsequent annealing. The porous nanostructure enhances the photoelectrochemical performance of the titanium dioxide nanorod array photoelectrodes under Uv-visible light illumination, where the highest photocurrent density was relatively about 10 times higher than that of the pristine titanium dioxide nanorod array. The formation of nanocavities mainly contributes to the enhancement of the photocurrent density by trapping holes inside to separate the charge carriers. The study demonstrates that ion implantation could be an effective approach to develop novel porous nanostructural photoelectrodes for the application of hydrogen production.

Engineering the interface and surface structures of semiconductor-based photoelectrodes for improved charge transfer dynamics and promoted water redox reaction kinetics is essential to achieve efficient photoelectrochemical (PEC) water splitting. In this work, α-Fe2O3 nanorods, successively coated with TiO2 and CoOx thin layers, were reported as the photoanode for solar-driven water oxidation. The obtained α-Fe2O3/TiO2/CoOx photoanode exhibits superior PEC performance as compared to bare α-Fe2O3, with a 3.3-time improvement in photocurrent density at 1.23 V vs reversible hydrogen electrode. This significant enhancement results from the formed heterojunction between α-Fe2O3 and TiO2 for the accelerated photogenerated charge separation and transfer as well as the passivated surface defects by the TiO2 overlayer for reduced charge recombination. Additionally, the existence of CoOx as the oxygen evolution catalyst significantly facilitates the surface reaction kinetics and thus reduces the overpotential for water oxidation. This study demonstrates a collaborative strategy of interface and surface engineering to design novel structures of α-Fe2O3 based photoanodes for highly efficient solar water oxidation.

Novel hybrid tetra-nitro-phthalocyanine Copper(II) (TNCuPc)/TiO2 films were successfully synthesized using a simple hydrothermal and solvothermal method. The successful deposition of TNCuPc on the TiO2 nanorods was confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The precursor concentration for hydrothermal deposition of TiO2 was optimized for light absorption and photoelectrochemical (PEC) water splitting. The hierarchical nanostructure exhibited increased light absorption ability and enhanced PEC properties. Moreover, the concentration of the precursor solution for TNCuPc deposition was also optimized for the performance of the TNCuPc/TiO2 hybrid nanostructure. The optimal sample achieved the highest photocurrent of 1.1 mA cm−2 and the highest IPCE of 51% at 610 nm.

In this study, a ternary nanostructured α-Fe2O3/Au/TiO2 film with integrating a crystalline α-Fe2O3 core, metallic Au nanoparticles (NPs), and an amorphous TiO2 overlayer is fabricated and examined as a photoanode for photoelectrocatalytic water splitting. Under simulated solar illumination, the as-prepared photoanode exhibits a four-fold increase (1.05 mA cm−2) in photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) relative to bare α-Fe2O3. Based on systematic investigations, it is proposed that Au NPs extract photoholes from the bulk of α-Fe2O3 core and then shuttle them to the outer TiO2 overlayer, and meanwhile, TiO2 overlayer efficiently captures and stores the photoholes and facilitates the hole injection into electrolyte. Thus, the remarkably improved photoelectrocatalytic water splitting performance of α-Fe2O3/Au/TiO2 photoanode is attributed to the significantly suppressed bulk and surface charge recombination due to the relayed pumping of the photogenerated charge carriers through the photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer.

Solar hydrogen conversion via photoelectrochemical water splitting is an important technology for energy and environment sustainability. Since the pioneering work of Fujishima and Honda in 1972, tremendous research on semiconductor-based photoelectrochemical water splitting has yielded better understanding of the processes as well as encouraging development of high efficiency photoelectrodes for solar hydrogen generation. Given the narrow band gap enabling excellent optical absorption, increased charge carrier density and accelerated surface oxidation reaction kinetics become the key points for improved photoelectrochemical performances for water splitting over α-Fe2O3 photoanodes. In this talk, some recent progresses in surface modified α-Fe2O3 for photoelectrochemical solar water splitting in our group will be introduced. By engineering the surface structures of α-Fe2O3 nanorods with AgxFe2-xO3, TiO2 and HfO2 overlayers, the surface charge recombination was greatly inhibited and the surface water oxidation kinetics were efficiently accelerated, resulting in remarkable enhancement in PEC water splitting performances.

Hematite (α-Fe2O3) has been extensively suggested as a superior photoanode for the photoelectrochemical (PEC) water splitting, owing to its natural abundance, high chemical stability, and theoretical solar-to-hydrogen (STH) efficiency (16.8%). Nevertheless, the recalcitrant electron−hole recombination resulting in poor charge separation and injection efficiency limits its PEC performance. Herein, a simple hydrothermal/atomic layer deposition (ALD) process was used to fabricate two types of HfO2 functionalized hematite (i.e., HfO2 overlayer and nanoparticles modified α-Fe2O3, HfFe-L and HfFe-P for short) photoanodes. It was revealed that HfO2 overlayer and nanoparticles could successfully passiviate the surface trap states of hematite, resulted in enhanced PEC performances for both HfFe-L and HfFe-P. More surprisingly, the photocurrent density of the HfFe-P reached as high as 1.21 mA cm-2 at 1.23 vs. RHE, with ~4.8 and ~3.3 fold enhancement as compared to that of the bare hematite and even HfFe-L, respectively. Such great PEC performance enhancement in HfFe-P was revealed to be attributed to the multifunction of HfO2 nanoparticles distributed on the surface of hematite nanorod arrays, which not only passivated surface defects to suppress the surface charge recombination by eliminating the surface trapping states, but can also facilitated the hole extraction from bulk of hematite to hematite/electrolyte interface.

Ta-doped hematite (α-Fe2O3) nanorod array films were successfully prepared on fluorine-doped tin dioxide (FTO) coated glass substrates via a facile solution growth process with TaCl5 as a Ta doping precursor. Under 1 sun illumination and at an applied potential of 1.0 V vs. Ag/AgCl, the Ta-doped α-Fe2O3 photoanode with optimized dopant concentration showed a photocurrent density as high as 0.53 mA cm(-2), which was about 3.5 times higher than that of the undoped sample. As demonstrated by Mott-Schottky and X-ray absorption spectroscopy measurements, considerable increase in photoelectrochemical (PEC) performance achieved for Ta-doped α-Fe2O3 nanorod films should be mainly attributed to the increased electron donor density induced by Ta doping. However, with superfluous Ta doping, the [110]-oriented nanorod structure was destroyed, which caused greatly restrained photoinduced holes transferring to the surface and retarded surface water oxidation reaction, leading to decreased PEC water splitting activity. This study clearly demonstrated that doping could be effective to enhance the PEC activity of α-Fe2O3 nanorods as photoanodes, while it is of great necessity to balance the trade-off between the electronic structure and nanostructure evolution by optimizing the dopant concentration, for increased donor density and meanwhile with the nanorod nanostructure well preserved for directed charge transfer.

The photoelectrochemical (PEC) water splitting activity of Nb and Ta-doped hematite (α-Fe2O3) nanorods was investigated with reference to electronic structures by in situ synchrotron x-ray absorption spectroscopy (XAS). Current density-potential measurements demonstrate that the PEC activity of α-Fe2O3 nanorods depends strongly on the species and concentrations of dopants. The doping of α-Fe2O3 nanorods with a low level of Nb or Ta can improve their electrical conductivity and thereby facilitate charge transport and reduced electron-hole recombination therein. The photoconversion effects of Nb and Ta-doped α-Fe2O3 by in situ XAS in the dark and under illumination revealed opposite evolutions of the spectral intensities of the Fe L-edge and Nb/Ta L-edge, indicating that charge transfer and a conduction pathway are involved in the photoconversion. Analytic in situ XAS results reveal that the α-Fe2O3 that is doped with a low level of Nb has a greater photoconversion efficiency than that doped with Ta because Nb sites are more active than Ta sites in α-Fe2O3. The correlation between PEC activity and the electronic structure of Nb/Ta-doped α-Fe2O3 is examined in detail using in situ XAS and helps to elucidate the mechanism of PEC water splitting in terms of the electronic structure.

ZnFe2O4 modified α-Fe2O3 nanorod array films were successfully synthesized by aqueous solution growth followed by a spin coating process and studied for photoelectrochemical (PEC) water splitting under solar light. Through the spin coating process, ZnFe2O4 was found mainly to exist at the top surface of the α-Fe2O3 nanorod arrays. After being modified by ZnFe2O4, the α-Fe2O3 nanorod film was identified as a suitable surface treatment, and showed an increased photocurrent density when used as the photoanode. It was demonstrated that ZnFe2O4 increased the charge carrier density and acted as the oxidation cocatalyst. These results suggested that surface treatment by ZnFe2O4 should be a practical strategy to improve the efficiency of α-Fe2O3 or other metal oxide photoanodes for PEC solar fuel conversion.